1. Field of the Invention
The present invention relates to a heating method and apparatus for cooking food, and more specifically to an improved heating apparatus employing a microprocessor and an alcohol sensor for controlling the heating operation. The present invention may advantageously be employed as a microwave oven.
2. Description of the Related Art
Recently, a microwave oven has been proposed that includes a microprocessor and an alcohol sensor to control heating more automatically and delicately. The quantity of alcohol gas generated from a food item to be cooked is detected by the alcohol sensor, and heating is controlled by the microprocessor based on the output of the alcohol sensor. For example, the heating time and the kind and amount of food to be cooked is determined from the sensor output. However, when heating operations are repeated in such a microwave oven, the alcohol gas generated from a previously cooked food may remain in the heating cavity, adversely affecting future cooking operations.
With reference to FIG. 12, during the time period A, a first heating operation is performed during which the quantity of alcohol gas detected by the alcohol sensor increases. Then after a time period B, the next heating operation is performed during a time period C. However, the amount of alcohol gas generated from the new food item cannot be detected and the heating operation cannot be performed properly because an amount of alcohol gas corresponding to voltage Vd, generated from the previous heating operation remains in the heating cavity at the beginning of time period C.
Therefore, to properly detect alcohol gas, it is necessary that the air in the heating cavity be alcohol-free at the beginning of a heating operation.
Examined Japanese patent application No. Shou 61-526 published on Jan. 9, 1986 discloses a cooking apparatus wherein air in the heating cavity that is potentially contaminated with alcohol is discharged by a fan disposed in the microwave oven immediately before every heating operation. However, from the time that food is placed in the heating cavity until the cleaning operation is completed, some alcohol gas may be generated by the food if the food contains a great amount of alcohol. In this case, the cleaning operation drives off alcohol gas which ought to be detected by the alcohol sensor. Therefore the true quantity of alcohol gas is not detected and the heating operation cannot be performed properly. Furthermore the cleaning operation is always performed, even when the cleaning operation is not necessary. For example, cleaning is not necessary when very little alcohol gas is left in the heating cavity. In this case, the unnecessary cleaning operation prevents the next heating operation from being performed immediately.